Research On Grid-connected Photovoltaic Micro Inverter

Facing with double threats of energy crisis and environment deterioration, cutting down the consumption of fossil fuel, developing clean energy has become the strategy that various countries in the world has chosen. clean energy meanly refers to those energy that reserves abundantly and it has less affected to the environment while using it, such as wind, solar and hydro energy, among these, rapid development of photovoltaic power generation has become the world trend. Among structures of the PV inverter, AC module has benefits that every module works individually, highly modularization, small size, high conversion efficiency, and has good prospects for development. Micro inverter is the core of AC module structure, therefore, the study of micro inverter is of important theoretical reference and engineering application value.In this paper, by comparison the first, the interleaved flyback topology is chosen to design a micro inverter, the speciality that working in the different modes has been analyzed, and DCM is chosen because its current source speciality, the working principle of average current control is analyzed, the flyback output current is modulated to be a half-sine wave current,and then a reversing bridge converted the current to be a sine-wave current. With regard to the problem that when the main switch turn off,the peak voltage and the power loss is high, which is caused by the leakage inductance, the active clamping technique is adopted, the un-complementary turn-on strategy between the main switch and the auxiliary switch is analyzed in detail, the recycling of the leakage inductance and the ZVS turn-on of the main switch is realized, the efficiency and stability of the micro inverter is improved.In this paper, the MPPT methods of PV module are researched, an improved variable step size conductance increment method is designed, it has fast tracking speed and high tracking accuracy, the dilemma of tracking speed and accuracy in traditional self-adaptation variable step size conductance increment method is solved, also DC bus voltage feed-forward is joined to solve the problem that when illumination suddenly drops the DC bus voltage may corrupt. The method combined with current loop which is aimed to improve the quality of grid current has carried on the MATLAB simulation. The simulation result shows the designed MPPT method has a good dynamic performance and steady-state precision.In this paper, a control strategy that improves the weighted efficiency is also designed, via route control and active clamp circuit control, the loss is reduced, the weighted efficiency is improved. The strategy combined with current loop has carried on MATLAB simulation, the feasibility of this strategy is analysed.Based on the theoretical analysis, a DSP 28035 is used as the core control chip, a 250 W micro inverter experimental prototype is designed and made, the design principle of hardware and software is introduced in detail. A DC voltage source is used as input, the Grid-connected experiments and analysis under average current control is completed, the Grid-connected current closed-loop control experiment and analysis is completed, the feasibility of this designing scheme is verified in the experimental result.